The liquid crystal display (LCD) technology has made a remarkable progress in the past years. Cellular phones, laptops, monitors, TV sets and even public displays based on LCD panels are presented on the market. The market of LCD is expected to keep growing in the near future and sets new tasks for researchers and manufacturers. One of the key growth sustainers is product quality improvement along with cost reduction.
LCD size has exceeded 100 inch by diagonal and it imposes stronger restrictions onto the quality of optical components. Retardation films should deliver a very small color shift and ability to provide higher contrast ratio at wide viewing angles in order to be used for high-quality viewing of large displays.
There are still some disadvantages of LCD technology which impact the quality of liquid crystal displays. One of disadvantages is a decrease of contrast ratio at oblique viewing angles. In conventional LCD the viewing angle performance is strongly dependent upon polarizers' performance. Typical LCD comprises two dichroic polarizers crossed at 90°. However, at oblique angles the angle between projections of their axes deviates from 90°, and the polarizers become uncrossed. The light leakage increases with increasing off-axis oblique angle. This results in low contrast ratio at wide viewing angle along the bisector of crossed polarizers. Moreover, liquid crystal cell placed between crossed polarizers makes leakage even higher.
Thus, modern technology requires development of new optical elements based on new materials with controllable properties. In particular, modern visual display systems require use of an optically anisotropic birefringent film that is optimized for the optical characteristics of an individual LCD module.
Various polymer materials are known in the prior art, which are intended for use in the production of optically anisotropic birefringent films. Optical films based on these polymers acquire optical anisotropy through uniaxial extension.
A triacetyl cellulose films are widely used as negative C plates in modern LCD polarizers. However, their disadvantage is a low value of birefringence. Thus, thinner films with high retardation value are desired for making displays cheaper and lighter.
Besides the stretching of the amorphous polymeric films, other polymer alignment techniques are known in the art. Thermotropic liquid crystalline polymers (LCP) can provide highly anisotropic films characterized by various types of birefringence. Manufacturing of such films comprises coating a polymer melt or solution on a substrate; for the latter case the coating step is followed by the solvent evaporation. Additional alignment actions are involved as well, such as an application of the electric field, using of the alignment layer or coating onto a stretched substrate. The after-treatment of the coating is set at a temperature at which the polymer exhibits liquid crystalline phase and for a time sufficient for the polymer molecules to be oriented. Examples of uniaxial and biaxial optical films production can be found in multiple patent documents and scientific publications.
In the article by Li et al, Polymer, vol. 38, no. 13, pp. 3223-3227 (1997) the authors noted that some polymers provide optical anisotropy which is fairly independent of film thickness. They described special molecular order of rigid-chain polymers on the substrate. The director of molecules is preferentially in the plane of the substrate and has no preferred direction in the plane. However, the described method has a technological drawback. After applying the solution onto a hot substrate, temperature was controlled at 60° C. to gently evaporate the solvent and dry the film for 60 min. After that the samples were dried at an elevated temperature of 150° C. for 24 h in a vacuum oven to remove any residual solvent. The last step severely restricts the product commercialization and does not allow using the plastic substrate for LCD manufacturing.
Shear-induced mesophase organization of synthetic polyelectrolytes in aqueous solution was described by T. Funaki et al. in Langmuir, vol. 20, 6518-6520 (2004). Poly(2,2′-disulfonylbenzidine terephtalamide (PBDT) was prepared by an interfacial polycondensation reaction according to the procedure known in the prior art. Using polarizing microscopy, the authors observed lyotropic nematic phase in aqueous solutions in the concentration range of 2.8-5.0 wt %. Wide angle X-ray diffraction study indicated that in the nematic state the PBDT molecules show an inter-chain spacing, d, of 0.30-0.34 nm, which is constant regardless of the concentration (2.8-5.0 wt %). The d value is smaller than that of the ordinary nematic polymers (0.41-0.45 nm), suggesting that PBDT rods in the nematic state have a strong inter-chain interaction in the nematic state to form the bundle-like structure despite the electrostatic repulsion of sulfonate anions. In the concentration range from 2 to 2.8 wt % a shear-induced birefringent (SIB) mesophase was observed.
The rigid rod water-soluble polymers were described by N. Sarkar and D. Kershner in Journal of Applied Polymer Science, Vol. 62, pp. 393-408 (1996). The authors suggest using these polymers in different applications such as enhanced oil recovery. For these applications, it is essential to have a water soluble shear stable polymer that can possess high viscosity at very low concentration. It is known that rigid rod polymers can be of high viscosity at low molecular weight compared with the traditionally used flexible chain polymers such a hydrolyzed poly-acrylamides. New sulfonated water soluble aromatic polyamides, polyureas, and polyimides were prepared via interfacial or solution polymerization of sulfonated aromatic diamines with aromatic dianhydrides, diacid chlorides, or phosgene. Some of these polymers had sufficiently high molecular weight (<200 000 according to GPC data), extremely high intrinsic viscosity (˜65 dL/g), and appeared to transform into a helical coil in salt solution. These polymers have been evaluated in applications such as thickening of aqueous solutions, flocculation and dispersion stabilization of particulate materials, and membrane separation utilizing cast films.
The present invention provides solutions to the above referenced disadvantages of the optical films for liquid crystal display or other applications.